Investigation of thyroid function in dogs treated with the tyrosine kinase inhibitor toceranib

Tyrosine kinase inhibitors are widely utilized in veterinary oncology for the treatment of mast cell and solid tumours. In man, these drugs are associated with thyroid dysfunction: however, to date only one study has investigated this in dogs. The aim of this study was to prospectively assess thyroid function in a group of dogs with cancer receiving toceranib. Thirty-four dogs were prospectively enrolled at two referral hospitals into two groups; those receiving toceranib with prednisolone and those receiving toceranib alone. Total thyroxine (TT4) and thyroid stimulating hormone (TSH) was monitored at regular time points during treatment. Follow-up data was available for 19 dogs. Overall, 12 incidences of elevated TSH occurred but none of these dogs had concurrent low TT4 concentrations. There was a significant difference in median TSH at week six compared with baseline. Hypothyroidism was not diagnosed in any patient during the study period. Patient drop-out was higher than anticipated which prevented the assessment of longer term toceranib administration on thyroid function. Toceranib therapy was not associated with hypothyroidism in this study but did result in elevations in TSH which confirms what has been previously reported. Toceranib should be considered to cause thyroid dysfunction in dogs and monitoring is advised.